Silica sols and use of the sols

ABSTRACT

Silica sols with a high content of microgel and particles with a specific surface area within the range 300 to 700 m 2  /g, preferably 400 to 650 m 2  /g. The sols can be prepared by acidification or a water glass solution and alkalization at a certain dry content, alternatively to a certain pH. The sols are particularly suitable for use as additives in papermaking in combination with cationic polymers and especially in combination with cationic acrylamide based polymers.

The present invention relates to new silica sols, to a process suitablefor preparation of the sols and to the use of the new sols in theproduction of paper. More particularly the invention relates to newsilica sols which have particles of a certain specific surface area andwhich have a comparatively high amount of so called microgel. The newsols are particularly suitable for use as additives in combination withpolymers in the production of paper.

Silica sols, which term is used herein for silica hydrosols, are aqueoussystems with very small silica particles which find use in a variety offields, among other things dependent on the particle size. In theproduction of paper silica based sols with very small, colloidal,anionic silica particles have found an increasing utilization during thelast few years. Silica sols are hereby used as additives to the stock incombination with cationic or amphoteric polymers, mainly for improvingretention and dewatering in the production of paper. In the Europeanpatent 41056 the use of colloidal silica sols in combination withcationic starch is for example disclosed. In the PCT applications WO86/00100 and WO 86/05826 combinations of silica sols, with particleswhere at least the surface groups contain aluminium, and cationicnatural polymers and cationic polyacrylamides, respectively, aredisclosed. The silica particles are generally stated to have a specificsurface area within the range of from 50 to 1000 m² /g. The sols whichare used commercially in paper production are of the type which havediscrete colloidal particles with a particle size usually of from about4 to about 7 nm, ie a specific surface area of from about 700 to about300 m² /g, and above all sols with particles having a specific surfacearea of about 500 m² /g have been used commercially. It has generallybeen considered that sols with particles of the above mentioned sizegive the best results and they have also been preferred with regard tostability. Quite generally it has been an aim that the silica solsshould be as monodisperse as possible, ie that the particles of the solsshould be discrete and non-aggregated and have as narrow particle sizedistribution as possible. At the preparation of sols it has thus beensought to avoid aggregation, ie formation of microgel. According to thePCT application WO 91/07350 silica sols with a certain degree ofmicrogel formation have been developed. These sols are based onparticles with a very high specific Surface area and have been foundespecially useful in combination with polymers in the production ofpaper. The sols are based on particles with very high specific surfacearea, from 750 to 1000 m² /g, preferably from 800 to 950 m² /g, and theparticles are surface modified with aluminium for stabilization of thehigh surface area.

According to the present invention it has been found that sols of silicabased particles, ie particles based on SiO₂, which have a specificsurface area within the range 300 to 700 m² /g and which containcomparatively high amount of microgel give very good effect as concernsretention and dewatering in papermaking. It has especially been foundthat sols with particles of the given specific surface area and contentof microgel according to the invention give substantially improvedeffect when they are used in combination with cationic acrylamide basedpolymers in comparison with previously used silica based sols withspecific surface area in the same range which substantially containdiscrete particles. The content of microgel, or aggregate, can becontrolled during the production of the sols.

The present invention thus relates to new silica sols, as furtherdefined in claims, and also relates to a process for the production ofthe sols and to the use of the sols.

The new sols are characteristic in that they have a high content ofmicrogel, ie a low S-value, and in that the sol particles have aspecific surface area within the range from 300 to 700 m² /g. The givenspecific surface area is measured by means of titration with NaOHaccording to the method disclosed by Sears in Analytical Chemistry28(1956):12, 1981-1983. The specific surface area is suitably within therange from 400 to 650 m² /g.

In contrast to known commercial sols with the above mentioned specificsurface areas which are used in papermaking the present sols have acomparatively high content of microgel and thus a low S-value. It isassumed that the microgel, the aggregates, to a substantial extent ispresent in the form of two- or three-dimensional structures, of moreless cloud-like formation, of aggregated primary particles. The S-valuefor the present sols is within the range from 15 to 40 percent by weightand preferably the S-value is within the range from 15 to 35%. The givenS-value has been measured and calculated in accordance with what isdisclosed by Iler, R. K. & Dalton R. L. in J. Phys. Chem. 60(1956),955-957. The S-value can be considered as a measure of the degree ofaggregate or microgel formation and a low S-value indicates a greaterpart of microgel and can also be considered as a measure of the SiO₂-content of the dispersed phase in percent by weight.

The particles in the present sols can be unmodified silica particles orbe silica particles which are surface modified with aluminium. Solswhich contain silica which is not aluminium modified are preferred. Foraluminium modified particles these are suitably modified to a degree offrom 2 to 25 percent, suitably from 3 to 20 percent. With a degree ofaluminium modification is meant the part of aluminium atoms which havereplaced silicon atoms in the surface of the particles. The degree ofaluminium modification is given in percent and is calculated on basis of8 silanol groups per nm². This is described by Iler, R. K. in Journal ofColloidal and Interface Science , 55(1976):1, 25-34. The present solssuitably have a dry content, calculated as SiO₂, of from about 3 toabout 40 percent by weight and the dry content is preferably within therange of from about 5 to about 30 per cent by weight.

It has been found that these new sols which contain anionic particlesare useful in the production of paper and similar products and that theyherein, in combination with cationic polymers give very good improvementof retention and dewatering. It has particularly been found that the newsols according to the invention which contain particles which are notaluminium modified give a substantial improvement when they are used incombination with cationic acrylamide based polymers in comparison withcommercially used such sols which have particles of the same size but inwhich the particles are substantially discrete. In addition to silicasols as defined in the appended patent claims the present invention alsorelates to a process for the production of the new silica sols and tothe use of the sols as stated in the appended patent claims.

The invention also relates to a process for the production of silicasols which have comparatively low S-values and which have a specificsurface area within the range of from 300 to 700 m² /g. According to theinvention silica sols are prepared starting from a conventional alkaliwater glass, potassium or sodium water glass, preferably sodium waterglass. The mole ratio of SiO₂ to Na₂ O or K₂ O, where Na₂ O and K₂ O inthe following will be given as M₂ O, in the water glass can, as per seknown, be within the range of from 1.5:1 to 4.5:1 and is preferablywithin the range of from 2.5:1 to 3.9:1. A diluted solution of the waterglass is utilized and this suitably has an SiO₂ content of from about 3to about 12 percent by weight, preferably from about 5 to about 10percent by weight. The water glass solution which usually has a pHaround 13, or above 13, is acidified to a pH of from about 1 to about 4.The acidification can be carried out in per se known manner by additionof mineral acids, such as for example sulfuric acid, hydrochloric acidand phosphoric acid or optionally with other known chemicals foracidification of water glass such as ammonium sulphate and carbondioxide. At addition of mineral acid the acidification is carried out intwo steps, a first step to a pH of about 8 to 9, whereafter a certainripening, ie a particle growth, is allowed to occur before furtheracidification to a pH of from about 1 to about 4. However, it ispreferred that the acidification is carried out by means of acid cationexchangers which among other things lead to more stable products andgive almost sodium free acid sols. The acidification is preferablycarried out by means of strongly acid cation exchange resins, forexample of sulfonic acid type. It is preferred that the acidification iscarried out to a pH of from about 2.0 to 4.0 and most preferably fromabout 2.2 to about 3.0. The acid sol obtained after acidification isthen made alkaline. The alkalization can be carried out withconventional alkali such as sodium, potassium or ammonium hydroxide. Itis, however, preferred that alkatization is carried out by addition ofwater glass. Potassium and sodium water glass, particularly sodium waterglass, with a mole ratio of SiO₂ to M₂ O as above described is used inthis alkalization step. The SiO₂ content of the water glass solutionsused for alkalization is suitably within the range of from about 3 toabout 35 percent by weight and preferably within the range of from 5 to30 percent by weight. The alkalization is suitably carried out to a pHof at the lowest equal to 7 and suitably to a pH within the range offrom 7.5 to 9. The alkalization is further suitably carried out to afinal mole ratio SiO₂ to M₂ O within the range of from about 20:1 toabout 75:1, suitably within the range of from about 30:1 to about 60:1.At the preparation of a sol as above the degree of microgel can beinfluenced in several ways and be controlled to the desired low value.The degree of microgel can be influenced by salt content, by adjustmentof the concentration at the preparation of the acid sol and at thealkalization since in this step the degree of microgel is influencedwhen the stability minimum for the sol is passed, at a pH of about 5. Byprolonged times at this passage the degree of microgel can be directedto the desired value. It is particularly suitable to control the degreeof microgel by adjustment of the dry content, the SiO₂ content, at thealkalization whereby a higher dry content gives a lower S-value. Bykeeping the SiO₂ content at the alkalization in the range of from 7.5 to5 percent by weight the S-value can be controlled to the given values 15to 40%. Another suitable way to control the degree of microgel is byadjustment of the alkalization to a certain pH and the above given pHvalues to which the alkalization is carried out controls the S-values tolower values at a lower pH. To obtain sols with S-values within therange 15 to 40% the pH at the alkalization is suitably controlled to therange 7.5 to 8.5. At alkalization to this pH-range a suitable SiO₂content is within the range from about 5 to about 6 percent by weight.The acid sol has particles with a high specific surface area, above 1000m² /g and usually around about 1300 m² /g. After the alkalization aparticle growth starts and thereby a decrease of the specific surfacearea. After the alkalization a growth process is thus carried out sothat the desired specific surface area is obtained. The desired decreasein surface area to the range 300 to 700 m² /g can be obtained by heattreatment. At heat treatment times and temperatures are adjusted so thatshorter times are used at higher temperatures. From a practical point ofview it is suitable to carry out heat treatment at temperatures up toabout 95° C. during about half an hour up to about 24 hours. When theparticles have obtained the desired specific surface area an aluminiummodification of the surface can be carried out if desired in order togive particles which have a better charge stability in acid environmentthan unmodified particles. The modification is an aluminium modificationand is carried out by means of an aluminate, sodium or potassiumaluminate, preferably sodium aluminate. The aluminium modification ofthe particle surface is carried out in per se known manner and to adegree of surface modification of 2 to 25%, particularly to a degree of3 to 20%, as mentioned above. According to the present process silicasols with dry contents of from about 3 to about 40 percent by weight,after optional concentration, can be prepared and the produced sols showvery good storage stability, ie they can be stored for several monthswithout substantial decrease of the specific surface area and withoutgel formation.

The new sols according to the invention are particularly suitable foruse in papermaking. The present invention relates also to this use ofthe sols. As mentioned by way of introduction it is well known to usesilica based sols in combination with cationic polymers in theproduction of paper, first of all in order to get improved retention anddewatering. The present silica sols are utilized in the same manner asearlier known for silica sols with anionic particles and they give, incombination with cationic polymers and amphoteric polymers, asubstantial improvement of the retention and dewatering in theproduction of paper. Even if arbitrary order of addition can be used itis preferred that the polymer is added before the sol. The sols can withgood effect be used within the whole pH range 4 to 10 in papermaking.The improved dewatering also results in that the speed of thepapermaking machine can be increased and, further, less water need to beremoved in the press and drying sections of the machine and asubstantially economically improved papermaking process is thusobtained. The substantially improved effect of the present sols with lowS-values in comparison with corresponding sols of higher S-values whenthey are used in combination with cationic acrylamide based polymersshould be especially emphasised.

The present invention thus also relates to a process for the productionof paper with the characteristic features defined in the claims. Thecationic or amphoteric polymers can be natural, ie based oncarbohydrates, or be synthetic. As examples of suitable polymers can bementioned cationic and amphoteric starch, cationic and amphoteric guargum, cationic and amphoteric acrylamide based polymers, cationicpolyethyleneimines, polyamidoamines and poly(diallyldimethyl ammoniumchloride). The polymers can be used singly or in combination with eachother. Cationic acrylamide based polymers are the preferred polymers foruse in combination with the present sols.

The amount of silica sol and polymer in the production of paperaccording to the present invention can vary within wide limits dependingon, among other things, type of stock, presence of fillers and otherconditions. The amount of sol should suitably be at least 0.01 kg/ton,calculated as SiO₂ on dry fibres and optional fillers, and is suitablywithin the range of from 0.05 to 5 kg/ton and preferably within therange from 0.1 to 2 kg/ton. The sol is suitably added to the stock atdry contents within the range of from 0.1 to 5 percent by weight. Theamount of polymer is to a high degree dependent on the type of this andother effects desired from this. For synthetic polymers at least 0.01 kgpolymer per ton, calculated as dry on dry fibres and optional fillersare usually used. Suitably amounts of from 0.01 to 3 and preferably from0.03 to 2 kg per ton. are used. For polymers based on carbohydrates,such as cationic starch and cationic guar gum, amounts of at least 0.1kg/ton, calculated as dry on dry fibres and optional fillers, areusually used. Suitably these are used in amounts of from 0.5 to 30kg/ton and preferably from 1 to 15 kg/ton. The weight ratio of cationicpolymer to sol, calculated as SiO₂, should usually be at least 0.01:1and suitably at least 0.2:1. The upper limit for the polymer is first ofall set by economical considerations and with regard to charges. Forpolymers of lower cationicity such as cationic starch, alone or incombination with other cationic polymers, very high amounts can thus beutilized, up to a ratio of 100:1 or higher, and the limit is mainlydecided by economical reasons. Suitable ratios of cationic or amphotericpolymer to sol, calculated as SiO₂, for most systems is within the rangeof from 0.2:1 to 100:1. The present sols can of course be used inpapermaking in combination with conventional paper additives such ashydrophobing agents, dry strength agents, wet strength agents etc..It isparticularly suitable to use aluminium compounds in combination with thepresent sols and cationic polymers since it has been found thataluminium compounds can give a further improvement of retention anddewatering. Any for use in papermaking known aluminium compound can beused, for example alum, polyaluminium compounds, aluminates, aluminiumchloride and aluminium nitrate. The amount of aluminium compound canalso vary within wide limits and it is suitable to use the aluminiumcompound in a weight ratio to the sol, calculated as SiO₂, of at least0.01:1 whereby the aluminium compound has been calculated as Al₂ O₃. Theratio suitably does not exceed 3:1 and is preferably within the rangefrom 0.02:1 to 1,5:1. The polyaluminium compounds can for example bepolyaluminium chlorides, polyaluminium sulphates and polyaluminiumcompounds containing both chloride and sulphate ions. The polyaluminiumcompounds can also contain other anions than chloride ions, for exampleanions from sulphuric acid, phosphoric acid, organic acids such ascitric acid and oxalic acid.

The silica sols and the polymers can be utilized in the production ofpaper from different kinds of stocks of cellulose containing fibres andthe stocks should suitably contain at least 50 percent by weight of suchfibres, based on dry material. The components can for example be usedfor stocks of fibres from chemical pulp, such as sulphate and sulphitepulp, thermomechanical pulp, refiner pulp or groundwood pulp from bothhardwood and softwood and can also be used for stocks based on recycledfibres. The stock can also contain mineral fillers of conventionaltypes, such as for example kaolin, titanium dioxide, gypsum, chalk andtalcum. The terms paper and papermaking which are used herein do ofcourse not include solely paper and its production but also othercellulose fibre containing products in sheet or web form such as pulpsheets, board and cardboard and their production.

The invention is further illustrated in the following examples which,however, are not intended to limit the same. Parts and percent relate toparts by weight and percent by weight unless otherwise stated.

EXAMPLE 1a)-1c)

In these examples different sols were prepared: 1a) Reference. Sol withS-value of about 53 containing non-aluminium modified silica particleshaving a specific surface area of 500 m² /g. This sol corresponds to acommercial sol developed on basis of the European patent 41056. The solwas prepared according to the following:

1275 g of water glass with an SiO₂ -content of 24.2% and a weight ratioSiO₂ :Na₂ O of 3.45 was diluted with 4045 g of water to an SiO₂ -contentof 5.8%. The water glass solution was ion exchanged in a column filledwith strong cation exchange resin (Amberlite IR 120) and diluted withwater to 5.41% SiO₂. 4000 g of the ion exchanged water glass was chargedto a reactor. While stirring well 354.2 g of water glass (5.8% SiO₂ ;SiO₂ :Na₂ O=3,45) were added to the ion exchanged water glass. The timeof addition was about 15 seconds. The alkalized solution was then heatedto 85° C. and heat treated at this temperature for 75 minutes. Afterfinished heat treatment the sol was cooled.

1b) According to the invention. Sol with an S-value of 31 containingnon-aluminium modified silica particles with a specific surface area of545 m² /g. The sol was prepared according to the following:

1625 of water glass with an SiO₂ -content of 24.2% and a ratio SiO₂ :Na₂O of 3.45 were diluted with 4075 g of water to an SiO₂ -content of 6.9%.The water glass solution was ion exchanged in an ion exchange columnaccording to Ex. 1a) and the ion exchanged water glass was diluted to6.49% SiO₂. 4600 g of the ion exchanged water glass were charged to areactor vessel. While stirring well 400 g of water glass (6.9% SiO₂ ;SiO₂ :Na₂ O=3,45) were added to the ion exchanged water glass. Thealkalized solution was then heated to 85° C. and heat treated at thistemperature for 60 minutes. After finished heat treatment the sol wascooled.

1c) According to the invention. Sol with an S-value of 21 containingaluminium modified silica particles with a specific surface area of 631m² /g. The sol was prepared according to the following.

A diluted water glass solution (SiO2:Na₂ O=3.4) was ion exchanged in acolumn and an ion exchanged water glass with an SiO₂ content of 5.36%was obtained. To 4000 g of this solution 80.6 g of water glass (22.2%SiO₂ ; SiO₂ :Na₂ O=3,41) were added. The addition time was about 15seconds. The alkalized solution was then heated to 75° C. and treated atthis temperature for 120 minutes. The sol was cooled and then H⁺-saturated cation exchange resin (Amberlite IR-120) was added in anamount to give a pH value of 7.2. The ion exchange resin was thenfiltered off. To 3770 g of the pH-adjusted sol 25.4 g of sodiumaluminate (Al₂ O₃ -content 25.5%) were added. Before the addition thesodium aluminate had been diluted with 225 g of water. The pH adjustedsol was heated to 45° C. before the addition and the addition time forthe aluminate was 60 minutes.

EXAMPLE 2

In this example the retention effect, retention of fibres and fillers,of sols 1a) and 1b) in production of paper was investigated. A standardstock, based on pulp with the composition 60% bleached birch sulphate+40% bleached pine sulphate to which had been added 30% of chalk asfiller and 0.3 g/l of Na₂ SO₄.10H₂ O, was used. The stock had aconcentration of about 5 g/l, a fine fraction content of 38% and a pH of8.1.

The retention effect, in this and following examples, was evaluated bymeans of a Britt Dynamic Drainage Jar at 800 rpm. This is theconventional retention test method used in the paper industry. The solswere used in varying amounts in combination with a cationic acrylamidebased polymer (Floerger Fp 4190 PG with 10 mole % cationic charges and amolecular weight of about 10 millions). The cationic polyacrylamide wasin all tests added in an amount of 0.8 kg/t and it was added before thesol. All given dosages, in this and following examples, are calculatedas dry on dry fibres and optional fillers. The results are shown in thefollowing table.

    ______________________________________                                        Sol 1a)                                                                       Ref.           Sol 1b) Retention                                              kg/t           kg/t    %                                                      ______________________________________                                        0.3                    48.5                                                   0.5                    51.9                                                   0.7                    53.9                                                   1.0                    58.0                                                   1.5                    61.9                                                                  0.3     57.2                                                                  0.5     63.7                                                                  0.7     73.5                                                                  1.0     76.1                                                                  1.5     78.7                                                   ______________________________________                                    

As evident a considerable improvement of the retention effect wasobtained when the cationic polyacrylamide was used in combination withsilica sol having high content of microgel, low S-value, according tothe invention in comparison with the effect when it was used incombination with a commercial sol with low content of microgel.

EXAMPLE 3

In this example the retention was evaluated in the same manner as in Ex.2 using sol c) according to the invention in comparison with a solaccording to the PCT application WO 91/07350, sol d), which like sol c)had an S-value of 21 but which had particles with a specific surfacearea of 897 m² /g. The stock was a standard stock with a concentrationof 5.2 g/1, a fines fraction content of 34% and a pH of 8.1. The samecationic polyacrylamide as in Ex. 2 was used and was added in an amountof 0.8 kg/t.

    ______________________________________                                        Sol 1d)                                                                       Ref.           Sol 1c) Retention                                              kg/t           kg/t    %                                                      ______________________________________                                        0.2                    43.9                                                   0.4                    58.5                                                   0.8                    73.0                                                                  0.2     47.5                                                                  0.4     71.6                                                                  0.8     72.1                                                   ______________________________________                                    

As evident as good retention can be obtained with aluminium modifiedsols with low S-value and a specific surface area of the order 600 m² /gas with aluminium modified sols with corresponding low S-value but withparticles having substantially larger surface area.

We claim:
 1. Silica sols having an S-value within the range from 15 to40 percent comprising anionic silica particles, said silica particlesbeing non-aluminum modified, and having a specific surface area withinthe range of from 300 to 700 m^(2/) g.
 2. The silica sols of claim 1wherein the silica particles have a specific surface area within therange of from 400 to 650 m^(2/) g.
 3. The silica sols of claim 1 whereinthe sol has an S-value within the range of from 15 to 35 percent.
 4. Thesilica sols of claim 1, wherein the sols have a dry content, calculatedas SiO₂, of from 5 to 30 percent by weight.
 5. A process for theproduction of paper, which comprises:(a) providing a suspensioncontaining cellulose fibers, and optional fillers; (b) adding to saidsuspension a cationic or amphoteric polymer retention and/or dewateringaid, said polymer being added in an amount of at least 0.01 kg/ton,based on dry fibers and optional fillers; (c) adding to said suspensiona silica sol comprising anionic silica particles, said silica particlesbeing non-aluminum modified and having a specific surface area withinthe range of from 300 to 700 m² /g, said silica sol having an S-valuewithin the range of from 15 to 40 percent, wherein said silica sol isadded to the suspension in an amount of at least 0.01 kg/ton, calculatedas SiO₂ on dry fibers and optional fillers; and (d) thereafter formingand dewatering the obtained suspension on a wire to form paper.
 6. Theprocess of claim 5 wherein the silica sol particles have a specificsurface area within the range from 400 to 650 m² /g.
 7. The process ofclaim 5, wherein the sol has an S-value within the range of form 15 to35 percent.
 8. The process of claim 5, wherein said polymer is acationic acrylamide based polymer.
 9. Silica sols having an S-valuewithin the range of from 15 to 40 percent comprising anionic silicaparticles, said silica particles being aluminum modified and having aspecific surface area within the range of from 300 to 700 m² /g.
 10. Thesilica sols of claim 9, wherein the silica particles are surfacemodified with aluminum to a degree of from 2 to 25 percent substitutionof silicon atoms.
 11. The silica sols of claim 9, wherein the silicaparticles have a specific surface area within the range of from 400 to650 m² /g.
 12. The silica sols of claim 9, wherein the sol has anS-value within the range of from 15 to 35 percent.
 13. The silica solsof claim 9, wherein the sols have a dry content, calculated as SiO₂, offrom 5 to 30 percent by weight.
 14. A process for the production ofpaper which comprises(a) providing a suspension containing cellulosefibers, and optional fillers; (b) adding to said suspension a cationicor amphoteric polymer retention and/or dewatering aid, said polymerbeing added in an amount of at least 0.01 kg/ton, based on dry fibersand optional fillers; (c) adding to said suspension a silica solcomprising anionic silica particles, said silica particles beingaluminum modified and having a specific surface area within the range offrom 300 to 700 m² /g, wherein said silica sol has an S-value within therange of from 15 to 40 percent and said silica sol is added to thesuspension in an amount of at least 0.01 kg/ton, calculated as SiO₂ ondry fibers and optional fillers; and (d) thereafter forming anddewatering the obtained suspension on a wire to form paper.
 15. Theprocess of claim 14, wherein the silica sol particles have a specificsurface area within the range of from 400 to 650 m² /g.
 16. The processof claim 14, wherein the sol has an S-value within the range of from 15to 35 percent.
 17. The process of claim 14, wherein said polymer is acationic acrylamide based polymer.